1. Field of the Invention
The invention relates to ionomers which have been modified with relatively low levels of a stearic acid moiety, particularly metal stearates and especially calcium stearate. The stearic-modified ionomers are especially useful when the ionomer is formulated for use as a golf ball core, center, one-piece ball, a soft golf ball cover, a mantle or in or as an intermediate layer between the center and the cover of a multi-layered ball.
2. Description of Related Art
Stearic acid and metal stearates have long been known as additives for many polymers. Typically they are used as process aids, and are referred to as lubricants dispersants release agents or plasticizers. They are generally used in small amounts. Their action may be external where, for instance, with nylon, ABS, polyester, and polystyrene they have been used, at low levels, to aid in metal release. Internally they may aid in dispersing additives in polymers, or, they may act as `lubricants` or `plasticizers` for the polymer itself. The words `lubricants` and `plasticizers`, particularly the latter, require specific definition, because they tend to be used as a catch-all, to describe effects on solid state properties such as stiffness, and/or melt properties, such as melt flow. Any art related to these additives must be examined carefully for what specific changes are brought about by the additive, since these words, particularly when used to describe stearic acid and stearates, are not used in a consistent way.
Typically, levels of 0.01 to 5 pph stearic acid or stearates are used in rubber and plastic formulations. In ionomers, 0.01 to about 1.0 pph zinc stearates has been utilized since the 1960s to facilitate the flow of ionomer resins in the molding process. U.S. Pat. Nos. 3,847,854 and 3,870,841 disclose the ability to plasticize the melt of ionic hydrocarbon polymers, by relaxing the ionic bonds. These plasticizers affect the melt and are not disclosed to affect the properties at normal use temperatures, since non-volatile plasticizers remain essentially as inert fillers and volatile ones are evolved after they have performed their process aid function. U.S. Pat. No. 3,847,854 discloses that with non-volatile plasticizers, which include calcium stearate, zinc stearate and stearic acid, amounts used should be no more than 6-7 weight percent, preferably less than 4 weight percent, so that melt plasticization, and not backbone plasticization is required. Backbone plasticization presumably results in a softer polymer. The ionomers listed include neutralized ethylene/(meth)acrylic acid polymers, in that the basic patent related to these, U.S. Pat. No. 3,264,272 is listed as describing typical ionomers. The disclosure however is concerned mainly with sulfonated ionomeric polymers. The disclosure of U.S. Pat. No. 3,870,841 is essentially similar with respect to calcium stearate, zinc stearate, and stearic acid.
U.S. Pat. No. 4,591,611 (Jenkins, et al.) discloses ionomeric polymers, including ethylene/methacrylic acid copolymers by patent reference, which contain about 5 to 125 parts of gilsonite per 100 parts of polymer which improve certain properties. The ionomeric polymers are preferentially sulfonated EPDM terpolymers. Optionally the compositions may contain from 5 to 40 parts of a melt plasticizer which is preferentially zinc stearate.
U.S. Pat. No. 5,312,857 discloses ethylene/carboxylic acid ionomers which contain from 10 to 100 parts, preferably 25 to 100 parts, of metal stearates, including zinc, calcium, barium, magnesium, sodium and aluminum. The compositions are disclosed as being useful for golf ball covers because such covers are cheaper and have no loss in properties as a result of the high level of inexpensive stearate, and even can show `similar or improved` coefficient of restitution (COR) and `similar or improved (decreased)` hardness. The changes in COR and reduction in hardness, if such changes occur at all, are however minimal. The essential aim appears to be to add a large amount of inexpensive filler while essentially maintaining the same properties. There is no disclosure of major effects on properties, and no disclosure of the compositions being useful for parts of a golf ball other than the cover. The examples presented in this patent suggest that the metal stearates do not negatively impact the otherwise known properties of the ionomers.
Ethylene/(meth)acrylic acid `hard` ionomers optionally containing a `softening` alkyl acrylate termonomer (`soft` ionomers), and blends of these, are well known for use as golf ball cover materials. Such ionomers are sold under the tradename SURLYN.RTM. ionomer resins, sold by E. I. du Pont de Nemours and Company. For the most part, existing ionomers exhibit a fixed relation between two key golf ball material properties, resilience and softness. Generally as resilience increases, so does hardness. The major drive in searching for improved ionomers in golf ball cover materials is to find ionomers which have improved resilience as measured by COR, yet have higher softness (measured, for instance by lower PGA Compression), or spin, relative to the COR. Hardness/softness can readily be changed by changing the ionomer composition, but deviations from the relatively fixed COR/PGA Compression line are difficult to achieve. Any composition which can achieve a positive deviation from this correlation is highly desirably, particularly for use in golf ball cores, centers, and also for golf ball covers.